Among the piezoelectric vibrators based on the bulk waves travelling in solid material are a piezoelectric vibrator used in various kinds of electronic apparatus as the clock source and a piezoelectric filter used in communication apparatus for extracting a frequency. The piezoelectric vibrator formed of lithium tantalate, among other materials, is used as a VCO (voltage control oscillator) or a broad band band-pass filter; the material lithium tantalate having a higher coefficient of electro mechanical coupling as compared with rock crystal, etc. representing the high efficiency of conversion between electric energy and mechanical energy. Among the piezoelectric vibrators, those made of the X plate of single crystal lithium tantalate are primarily vibrated by thickness shear vibration, and exhibit, depending on a direction of cutting, improved temperature characteristics in the resonance frequency.
Japanese Patent Laid-Open Application No. S59-182617, for example, discloses a strip-type thickness shear mode vibrator using the X plate lithium tantalate for improved frequency temperature characteristics. For improving the frequency temperature characteristics of the vibrator, especially to minimize a secondary temperature coefficient as well as a primary temperature coefficient, it is cut out so that the longitudinal direction of the vibrator lies, with the X-axis used as rotation axis, in a direction +48°±4° clockwise from the Y-axis (hereinafter, such direction is referred to simply as, e.g. +48°±4° from Y-axis), and a ratio Le/H of the length Le of an exciting electrode to the thickness H of the vibrator is not higher than 3.8. The terminology strip-type vibrator means a vibrator of strip shape, the cross sectional area of which being rectangular, and provided with exciting electrode disposed covering the entire width.
Japanese Patent Laid-Open Application No. H3-3514 teaches a vibrator which has an advantage of easy and precise processing, where the longitudinal direction forms an angle of +57° from the Y-axis. By so doing, the longitudinal direction coincides with cleavage direction of lithium tantalate.
FIG. 17 shows a conventional piezoelectric vibrator. Vibrator 50 includes vibrator element 51 formed of the X plate of single crystal lithium tantalate, exciting electrodes 52 provided opposed on the front and the reverse surfaces of the vibrator element, connection electrode 53 provided for connection with external devices, and lead electrode 54 for electrically connecting exciting electrode 52 with connection electrode 53.
In the conventional strip-type vibrator formed of the X plate lithium tantalate piezoelectric material, with its longitudinal direction forming an angle of +48° from the Y-axis, as shown in FIG. 17, the ratio Le/H has been set not to be higher than 3.8 for the purpose of improving the temperature characteristics. As the result, an area for exciting electrode can not be sufficiently large, resonance impedance increases and mechanical quality coefficient Q deteriorates. These problems have become increasingly significant in the face of the decreased thickness, decreased width dimension, decreased length of exciting electrode and decreased width dimension of exciting electrode of a vibrator caused as the results of introduction of higher frequencies.
If the angle of longitudinal direction of a vibrator is other than +48° from Y-axis, the vibrator exhibits different temperature characteristics. Conditions required for improving the temperature characteristics with a vibrator whose longitudinal direction is +57° from Y-axis have been unknown.